1. Field of the Invention
The present invention relates to an image processing method and image processing device by volume rendering.
2. Description of the Related Art
Hitherto, a projected image has been acquired by projecting virtual ray into a three-dimensional image obtained with a computed tomography (CT) apparatus, a magnetic resonance imaging (MRI) apparatus, or the like. As a processing for obtaining such a projected image, volume rendering has been widely employed. As the volume rendering, there are known, for example, MIP (Maximum Intensity Projection) processing wherein maximum voxel values are extracted in a projecting direction and are projected, MinIP (Minimum Intensity Projection) processing wherein minimum voxel values are extracted and projected, a ray casting method wherein a virtual ray is projected in a projecting direction and a reflected light from an object is calculated, etc.
FIGS. 31A to 31D are explanatory drawings of the MIP processing, and show a relationship between 3D (three-dimensional) data corresponding to voxel values of a rendering object and maximum values selected as data for display. In the MIP processing, since a maximum value of the 3D data on the projection line shown by an arrow in each figure is used as the display data, 4, 8, 8, and 8, each of which are maximum values of the 3D data, are used as the display data in FIGS. 31A, 31B, 31C, and 31D, respectively.
FIG. 32A shows a Raycast image, and FIG. 32B shows an MIP image. The Raycast image shown in FIG. 32A is one kind of volume rendering image, and pixels are determined by accumulating reflected light-s from a plurality of voxels on a virtual ray. Therefore, it is effective in rendering of outlines and a graphical image is obtained. Moreover, in the case that the virtual ray passes between voxel data, calculation may be conducted based on not the voxel data themselves but information obtained by interpolating the voxel data.
On the other hand, the MIP image shown in FIG. 32B is effective in objectivity and a high speed calculation is possible, since, as mentioned above, the pixels are determined by selecting a single voxel on a virtual ray and the voxel values are rendered as they are. Therefore, the MIP images are frequently used in the rendering of blood vessels. Sometimes interpolated voxel values are used in MIP processing, and a plurality of voxels are referred to, but there is no difference in the fact that only the information on a single point on the virtual ray is used. However, sometimes it becomes difficult to render an organ having no characteristic in the voxel values.
FIGS. 33A and 33B are drawings for illustrating the situation, in an MIP image, of a portion where a bloodstream 52 is obstructed by a calcified region 50 attached inside a blood vessel. Moreover, FIGS. 33A and 33B show the cases that the same portion of the blood vessel is observed in the directions 90 degrees different from each other.
In the MIP image shown in FIG. 33A, a size of the calcified region 50 having a high CT value in the blood vessel can be ascertained. However, the bloodstream 52 in a stenotic portion 51 obstructed with the calcified region 50 cannot be measured correctly in some cases. Moreover, in the MIP image shown in FIG. 33B, the observation of the bloodstream 52 becomes difficult since the calcified region 50 becomes an obstacle. The bloodstream 52 cannot be observed even when the bloodstream 52 actually positions at the back of or in front of the calcified region 50.
FIG. 34 is a drawing for explaining characteristics of voxel values profile along a virtual ray, showing a change in voxel values on the virtual ray at the portion where a calcified region having a high CT value exists in a blood vessel. On the virtual ray, the voxel values corresponding to the calcified region have large values and show sharp-peaked values. On the other hand, the voxel values of the bloodstream have small values and show smooth-shaped values.
Therefore, in the MIP image, since the maximum value of the voxel values on the virtual ray is directly displayed, when a blood vessel having a calcified region is observed, the calcified region with a large voxel value is displayed, and thus the bloodstream positions at the back of or in front of the calcified region cannot be displayed.
FIG. 35 is a drawing for illustrating a solution of the related art when the bloodstream positioning at the back of or in front of the calcified region is observed in the MIP image. As shown in FIG. 35, replaced volume data is generated by replacing the CT values of the calcified region with some values (e.g., volume data of air). Accordingly, the voxel values corresponding to the calcified region are lowered so as to display the bloodstream. Alternatively, substantially the same effect is obtained by removing a region corresponding to the calcified region from the rendering object. However, in the above two methods, it is necessary to perform a region extraction processing to specify the calcified region in advance.
That is, in the solution of the related art, at the pre-stage of the volume rendering, a calcified region is detected using a region extraction method with a threshold value or other algorithms. Then, using the result of the region extraction, the volume data is modified (the calcified region is removed) or mask data is generated (non-rendering region is designated using mask volume) to enable the display of the bloodstream.
FIG. 36 is a flow chart showing calculation of each pixel on the screen in a ray casting method of the related art. In the ray casting method, the following calculation is performed for all the pixels on the screen. First, from the projection position, a projection starting point O(x,y,z) and a sampling interval ΔS(x,y,z) are set (Step S201).
Next, a reflecting light E is initialized as “0”, a remaining light I as “1”, and a current calculation position X(x,y,z) as “O” (Step S 202). Then, from voxel data in the neighbor of the position X(x,y,z), an interpolated voxel value V of the position X is obtained (Step S203). In addition, an opacity a corresponding to the interpolated voxel value V is obtained (Step S204) In this case, a function of α=f(V) is prepared beforehand (Step S 212).
Next, a color value C corresponding to the interpolated voxel value V is obtained (Step S205). Then, from voxel data in the neighbor of the position X(x,y,z), a gradient G of the position X is obtained, and from a ray direction X−O and the gradient G, a shading coefficient β is obtained (Step S206).
Next, an attenuated light D (D=I*α) and partial reflecting light F (F=β*D*C) at the position X(x,y,z) are calculated (Step S207). Then, the reflecting light E and the remaining light I are updated (I=I−D, E=E+F) (Step S208).
Next, it is determined whether or not X reaches a final position, and whether or not the remaining light I is “0” (Step S209). When X is not the final position and the remaining light I is not “0” (no), ΔS(x,y,z) is added to X(x,y,z), the current calculation position is moved on (Step S210), and the processes of and after Step S203 are repeated. On the other hand, when X reaches the final position or the remaining light I is “0” (yes), calculation is finished with the reflecting light E being used as the pixel value of the pixel under calculation (Step 211).
FIG. 37 shows a flow chart for calculating each pixel on the screen in an MIP processing of the related art. In the MIP processing, the following calculation is performed for all the pixels on the screen. First, from the projection position, a projection starting point O(x,y,z) and a sampling interval ΔS(x,y,z) are set (Step S221).
Next, a maximum value M is initialized as a minimum value of the system and a current calculation position X(x,y,z) as “O” (Step S 222). Then, from voxel data in the neighbor of the position X(x,y,z), a interpolated voxel value V of the position X is obtained (Step S223).
Next, the maximum value M and the interpolated voxel value V are compared (Step S224). When the maximum value M is smaller than the interpolated voxel value V (yes), the interpolated voxel value V is assigned to the maximum value M as a new Maximum value (Step S225). Then, it is determined whether or not the current calculation position X reaches a final position (Step 226). When the current calculation position X is at the final position (no), ΔS(x,y,z) is added to X(x,y,z), the current calculation position is moved on (Step S227), and the processes of and after Step S223 are repeated. On the other hand, when the current calculation position X reaches the final position (yes), the maximum value M is used as the pixel value of the pixel under calculation (Step 228).
Moreover, in U.S. Pat. No. 6,205,350, second volume data not containing an obstructing region is generated, the maximum value in the second volume data is obtained, and a value in the original volume data at the position corresponding to the position of the maximum value is used for rendering.
However, in the above methods of the related art, obstructing regions such as calcified regions are removed by the replacement of volume data. Hence the information of the obstructing regions themselves is completely lost. Moreover, it is difficult to exclude exactly only the obstructing region and to render the bloodstream correctly. Furthermore, since an extracted region is designated in voxel units, aliases may arise at the boundary of the region, which results in deterioration of the image. In addition, retention of mask information and second volume data may cause an unnecessary load to the memory, and when the volume data is modified, the comparison with the original date becomes difficult. Additionally, the extraction of individual obstructing regions takes much time and largely depends on subjective factors of a user. In particular, since the extraction depends on the subjective factors of the user, reproducibility by each user is low, which results in lack of universality as objective diagnostic information. Therefore, there is a problem that it is difficult to use the methods at actual diagnosis and hence actually, they are not so widely employed.
FIGS. 38A, 38B and 38C are drawings for illustrating the problems in the MIP image of the related art. In the method of the related art, as shown in FIGS. 38A and 38B, a calcified region 61 is removed in order to observe a bloodstream 60 at the back of and in front of the calcified region 61. In that case, a portion 62 where the bloodstream 60 exists is also removed. Moreover, in the method of the related art, the calcified region 61 is not displayed at all, and hence it becomes difficult to determine a diseased part. Also a necessary region is removed frequently, and hence reliability decreases.
In this case, as shown in FIG. 38C, information of a portion 63 which is an imprint of the removed calcified region is necessary. Particularly, information of an outline portion 64 of the calcified region is required. That is, when only the outline of the calcified region is displayed without displaying the filling of the calcified region, the display is effective for diagnosis.
In this regard, since the calcified region is a three-dimensional region, the boundary surface of the region constitutes a curved surface in a three-dimensional space. Therefore, when the calcification is rendered by a mask application or a volume modification of the related art, each pixel of an image represents an intersection of a virtual ray and the three-dimensional boundary surface, the intersection constituting the pixel, so that a two-dimensional outline cannot be represented. On the other hand, when diagnosis is conducted while viewing the image, for the calculation of images, information of the two-dimensional outline portion of the calcified region and the neighbor of the calcified region, particularly at the back of and in front of the calcified region is necessary. With regard to the two-dimensional outline portion, when only the portion can be rendered where the virtual ray grazes the rim of the calcified region three-dimensionally, the rendering is effective for diagnosis.